Speed up webserver start up in Kube tests (#19710)
Speed up webserver start up in Kube tests (#19710)

Thanks to a previous change to not load provider hooks too early we can
take advantage of the "preload-app" feature of Gunicorn to load the
application once in the main gunicorn process before the workers are
forked off.

This change makes the webserver start up (time to serving first request)
go from 20s to 5s.

(The reason we don't just do this blindly everywhere is that it would
mean plugins are loaded at start only, and is a change in behaviour. But
in tests this is fine.)
danielvdende/incubator-airflow,nathanielvarona/airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,apache/airflow,cfei18/incubator-airflow,lyft/incubator-airflow,apache/airflow,lyft/incubator-airflow,bolkedebruin/airflow,cfei18/incubator-airflow,nathanielvarona/airflow,nathanielvarona/airflow,lyft/incubator-airflow,mistercrunch/airflow,apache/airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow,bolkedebruin/airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,Acehaidrey/incubator-airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,mistercrunch/airflow,nathanielvarona/airflow,mistercrunch/airflow,apache/airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow,apache/airflow,lyft/incubator-airflow,bolkedebruin/airflow,nathanielvarona/airflow,apache/airflow,mistercrunch/airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow